Step1
The first thing that you're going to need to do when making a computer parts list for a custom computer is to decide on the motherboard that you want. When looking at motherboards, you should take into account the amount of memory that it can hold and the type of processor that it supports. You should also have a general idea of the type of video card and any other accessories that you want to use in order to make sure that the motherboard you pick will support them.Another consideration for the motherboard is the size of case you will be using; the most common case size is ATX, but in case you already have a case which is another size then you should make sure the motherboard that you pick will fit into it. If you're going to be buying a new case, you can just buy a case that will hold whichever motherboard you choose.
Step2
Once you have your motherboard picked out, begin looking at the processor that you want. See which type of socket that your motherboard has, sorting CPU's by socket type so that you know the one you choose will fit. You should also look at the FSB rating of your motherboard so that you'll know the limitations that need to be adhered to when picking out a processor. This way you'll know that you don't have a processor that's too powerful for your motherboard to support.
Step3
Next you should move on to RAM, keeping in mind that your motherboard will only support certain types of memory. Not only will you need to see if the motherboard will support the physical size and shape of the RAM that you choose, but you should keep the voltage of the memory in mind as well. Look at the maximum capacity of memory that your motherboard will support, then pick out the amount of memory that you want to buy (sorting by voltage and type, of course.)
Step4
After you've picked out your processor and memory, the only other motherboard-dependent considerations to keep in mind will deal with its connections. If your motherboard doesn't have SATA capabilities then you won't be able to use an SATA hard drive, and if it doesn't support PCI express connections (or PCI express connections of the appropriate type) then you won't be able to use expansion cards that require specific types of PCI express slots. When looking at hard drives, video cards, sound cards, or any other accessories then you should make sure that you'll be able to connect them to your motherboard using the connections that you have available.
Step5
Once you've picked out your motherboard, RAM, processor, and other parts, all that remains is to choose a power supply. Take care to see how much power your motherboard and accessories are going to need and try to pick a power supply that will provide at least slightly more than that. This will make sure that you have enough power in case one or more of your components ends up requiring a little more power than what's listed in the specifications.
Step6
When you have chosen all of your components and your power supply, all that's left is to make sure that you have enough ventilation or sufficient cooling to keep your processor, video cards, and memory from overheating. This can include one or more fans or a liquid cooling system, depending on your preferences. If you don't have a case already, you should also pick out a case that will support the fans or cooling system that you want as well as the motherboard that you have picked out.
Step7
When you've chosen all of your computer parts, either write down the names and models of the various parts or save the list as a wish list on the website you were using to pick out what you need. You can then either purchase all of the parts at the same time, or one at a time to better suit your budget.
Novice 
